The invention relates to a method for monitoring the quality of a moving web of multicolor print during a printing process, comprising monitoring the mutual location of tho various colors on the basis of marks arranged on the web of print and further monitoring the location in longitudinal direction and transversal direction of the web of print in relation to at least one printing press. Such a method is known, for instance from the U.S. Pat. No. 5,018,213.
During a printing process it is of great importance that the quality of the print is continuously monitored as printing is a dynamic process, wherein over time deviations of the various set parameters may occur. For instance the properties of the web of material to be printed, in general paper may vary with the temperature and humidity in the room where the printing process is performed whereas these properties may further differ from one roll to another. Furthermore, the settings of the printing press may also shift, while variations in the supply of printing ink may also lead to variations.
In particular, in multicolor printing continuous monitoring is of great importance since such printing is performed in a plurality of subsequent printing steps in which one color is printed each time. It is then, of course, very important that the colors are printed on the web in the correct mutual relationship during subsequent printing steps since the formation of mixed colors depends on dots of the correct colors being printed in precisely determined ratios at precisely determined locations. Not only the location of the dots of the various colors in relation to each other is of importance, but also the location of a printed text or image in relation to the edges of the web. When this location is not correctly chosen, there is a risk that parts of the text or image may be lost during subsequent cutting of the print.
In the past monitoring of print was generally performed manually. To this end a completed example of the print was regularly checked, and on the basis of errors determined therein the required adjustments to the various settings were performed. This monitoring was time consuming. Moreover, the time which elapsed between an error being detected and the same being corrected was such that a relatively large quantity of print of low quality was made, therefore a large loss of production could occur. Thus, there exists a need for an apparatus and method for monitoring the printing process.
In the above-mentioned U.S. Pat. No. 5,018,213 a method is described in which by means of a digital camera an image is recorded of marks in various colors which are printed in a fixed and predetermined pattern on the web next to the actual print. From the images thus recorded the mutual location of the marks in the various colors is determined, after which in case of possible deviations from the required location an error signal is generated. This error signal may be used for performing the necessary adjustments. For recording the images the print is periodically illuminated by means of a stroboscope, whereby the print is "frozen" as it were in relation to the camera. This method has the drawback that the CCD chip functions in interlaced mode and so called "smearing" may occur in the image when the images move fast, so that the measurement may be inaccurate. The camera that is used has a fixed focal distance, so that the web of paper has to be guided such that it passes the camera at the right distance. To that end use is made of guide rolls, so that the ink has to be sufficiently dry when passing the camera. The camera is therefore arranged downstream of a drying street, which result in a long control loop.
No mention is made in this document of monitoring the location of the printed text or images in relation to the edges of the printed paper web. From other publications however, it is already known to measure the location of a web of paper in relation to the printing press by means of for instance two separate photoelectric sensors, one for detecting the position of the edge of web of paper and another one for detecting a separation mark between two subsequent images of the print.